This invention relates to vapor compression cycle devices, and more particularly to devices of such type which employ a multi-component working fluid mixture, and to a method of modulating the capacity of such devices to absorb and deliver heat.
Vapor compression cycle devices employing multi-component working fluids have been disclosed in the following copending patent applications, all of which are assigned to the same assignee as the present application: Ser. No. 926,510, filed July 20, 1978 now U.S. Pat. No. 4,217,760; Ser. No. 927,032, filed July 24, 1978 now U.S. Pat. No. 4,218,890; and Ser. No. 929,339, filed July 31, 1978 now U.S. Pat. No. 4,179,898. Each of these devices includes means to effect a modulation in device heating or cooling capacity in response to variations in demand. The present invention also includes means to modulate device capacity, and additionally, includes means for effecting a rapid transition from high to low capacity operation.
Each of the vapor compression cycle devices disclosed in the above-cited patents employs a multi-component working fluid mixture containing both low and high boiling point components and a pair of liquid accumulators connected through a flow restricting device to enable a modulation of device capacity. Due to equilibrium relationships between the working fluid vapor and liquid contained therein, the liquid in the first accumulator which flows from a condensing heat exchanger is enriched with the low boiling point component of the working fluid mixture, and the liquid in the second accumulator, which is connected to the inlet of a system compressor, is correspondingly enriched with the high boiling point component of the mixture. The capacity of the device to transfer heat is increased by adjusting the flow restricting device to allow a greater flow of the low boiling point component enriched liquid from the first accumulator to the second. The composition of the liquid in the second accumulator is thereby altered, resulting in a corresponding change in the vapor pressure or density at the compressor inlet. This accordingly increases the molar flow rate through the compressor (here in termed the flow rate of compression) which thereby increases the capacity of the device to absorb and deliver heat.
The capacity of each of the devices disclosed in the above cited patents is decreased by restricting the flow of liquid from the first accumulator to the second and by depleting the concentration of the low boiling point mixture component in the liquid contained in the second accumulator through evaporation. The evaporative process involves the transfer of heat from superheated vapor at the surface of the accumulator liquid. Although this process is effective, it requires an undesirably long time to switch from high to low capacity operation. For example, assuming a heat transfer coefficient of 20 BTU/hr ft.sup.2.degree. F., vapor superheat of 20.degree. F., a refrigerant liquid density based on Freon-22 fluorocarbon refrigerant of 80 lbs/ft.sup.3, and an evaporation enthalpy of 80 BTU/lb, an accumulator with a typical six inch head of fluid might require an eight hour switching time.
An additional problem common to many vapor compression cycle devices is the removal of lubricating oil from the compressor by the working fluid flowing therethrough, and the resultant collection of the oil in a liquid accumulator. More specifically, as the working fluid of a device circulates through a compressor it carries off lubricating oil in the form of oil mist through the compressor outlet. Additionally, provided sufficient temperature and pressure conditions are present, vaporized oil may be carried off by the working fluid. This removed oil collects in the liquid accumulator of the device, which can result in a depletion of compressor lubricating oil unless the oil-bearing liquid is recirculated. During the operation of the compressor, this depletion of oil is partly countered by the flow of working fluid which provides the compressor with a certain amount of lubrication. However, when the flow of fluid is interrupted such as during system startup, the lack of lubricating oil can cause serious compressor malfunctions.
Conventional vapor compression cycle devices have attempted to prevent compressor oil depletion by recirculating the oil-bearing accumulator liquid through the compressor. Inasmuch as the type of compressor typically employed in these devices operates to compress gases, it cannot accommodate the circulation of a substantial volume of liquid, which is relatively incompressable, without damaging the compressor. Accordingly, prior devices used to recirculate accumulator liquid and to thereby prevent oil depletion typically by limiting flow of oil-bearing liquid to the compressor to a trickle by including a specifically designed compressor inlet tube in the accumulator. The inlet tubes are in the form of a J-tube entering from above the accumulator, or a standpipe. In both types, an open end at the tube inlet extends into a vapor region of the accumulator, and a small aperture is provided below the level of the accumulator liquid to allow a limited amount of liquid to be extracted and sent directly to the compressor inlet admixed with working fluid vapor.
However, the modulation of capacity in a device as described above requires the varying of accumulator liquid level such that at a high liquid level corresponding to a high device capacity the liquid may overflow into the vapor inlet for the compressor, causing damage thereto as noted above. Conversely, a device operating at a low capacity could have a liquid level too low to allow the oil-bearing liquid to flow into the liquid extracting aperture in the compressor inlet tube. Thus, the conventional J-tube and standpipe designs do not present a practical solution to the oil depletion problem in capacity modulating devices such as those described in the above-cited patents.
Finally, a vapor compression cycle device should preferably have means to control evaporator superheat in order to improve the device performance. Superheat is the temperature of a vapor above that required for the evaporation thereof. A large superheat is often inefficient, and means for controlling it are required for optimal system performance.
Accordingly, it is an object of the present invention to provide a new and improved vapor compression cycle device.
Another object of the present invention is to provide a vapor compression cycle device including new and improved means for switching from high to low thermal capacity operation.
Another object of the present invention is to provide a new and improved vapor compression cycle device in which depletion of lubricating oil from the compressor is avoided.
Another object of the present invention is to provide a new and improved vapor compression cycle device which is adapted for enabling variable control of device capacity and of evaporator superheat.
Still another object of the present invention is to provide a new and improved method of operating a vapor compression cycle device whereby the time required to switch from a high device thermal transfer capacity to a low thermal transfer capacity is decreased, and evaporator superheat is controlled.